1. Field of the Invention
This invention relates to analogue-to-digital (A-D) converters, and especially to sigma-delta analogue-to-digital converters.
2. Background of the Invention
Sigma-delta A-D converters include quantising means for producing a digital output, oversampled relative to the signal bandwidth, and a feedback loop for feeding back a signal derived from the digital output to be combined with the analogue input for input to filter means, the output of the filter means being connected to the quantising means: this is in order to shape the quantisation noise to reduce it in a desired bandwidth (GB-A-2,232,023).
A typical implementation of such a sigma-delta converter is shown in FIG. 1, this having a second order filter and bandpass characteristic. Each part of the loop filter consists of parallel resonant circuits of an inductor and capacitor in parallel, L.sub.1 C.sub.1, L.sub.2 C.sub.2, in series with resistors R.sub.1, R.sub.2. Buffers 1-3 provide isolation between the filter stages and between the filter and the quantising means 4 and between the summing node 5 and the analogue input. The feedback loop includes a digital-to-analogue converter (D-A) 6.
The amplitude and phase of the voltage across each stage of the filter is shown in full line in FIGS. 2a, 2b. The effect of the series resistors R.sub.1, R.sub.2 is to add a uniform step FIG. 2a (shown dashed) to the amplitude response FIG. 2a (also shown dashed) of the resonant circuit and, more importantly, to reduce the phase shift of .+-.90.degree. produced at low and high frequencies (FIG. 2b, the dashed curve showing the phase shift if the resistors R.sub.1, R.sub.2 were replaced by short circuits). The reduction in phase shift reduces the tendency to instability which could arise if the combined phase shift of the two filter stages approached 180.degree., thus turning negative feedback into positive feedback.
A problem with such filter stages is that the buffers 2-3 and summing node 5 inherently have input and output capacitances to earth (FIG. 1, shown dashed) which load the tuned circuit. This degrades the Q of the resonant circuits, since some energy is exchanged between the input and output capacitance and the inductance L.sub.1 via the resistors R.sub.1, R.sub.2, so that energy is dissipated in the resistors. The reduction in the Q factor reduces the peak of the amplitude response and thus degrades the notch in the quantisation noise produced by the sigma-delta modulator and hence the signal-to-noise ratio which can be achieved.
An analagous problem arises in the case of sigma-delta converters with series resonant circuits connected in parallel with resistive means (not shown) to modify the phase response of the resonant circuits.
Another problem which is encountered by both band-pass and base-band sigma-delta A-D converters is that the presence of resistive means in the first stage of the filter imposes a certain bandwidth requirement on subsequent filter stages.